Dear readers: My sister confessed she is in love with me & I don't know what to do

My sister is in love with me. I am 3 years older than her at 20. I never expected this to happen although we've always been very close. Recently she has been cold towards me and ignores me a lot. Even when I asked her what was wrong she wouldn't respond properly. She locks herself in her room and cries constantly.
Our parents are divorced and we live with mom who is too busy with her work to take care of us. She isn't home enough to see the condition my sister is in. Then one day I noticed my sister had even resorted to cut her wrists so i decided to force her to tell me what was wrong and she ended up kissing me and telling me that she loved me in the wrong way. I was too shocked to do anything. I've never thought about her in any way other than family though. I don't want to hurt her. What should I do please help?